In developed countries, populations having immunity against HSV have recently decreased, and hence, they are experiencing serious problems in the increase of HSV infections such as herpes genetalis, neonatal herpes infection and herpes encephalitis in these countries. In order to prevent such HSV infections, vaccination is useful, and some vaccines have already been proposed such as an attenuated vaccine comprising attenuated HSV and an inactivated vaccine containing HSV DNA. It is known, however, that associated with HSV are problems such as latent infection and carcinogenicity, and the conventional attenuated vaccine and inactivated vaccine possess such side effects and hence are not preferable nor practical.
Cells infected by HSV produce several glycoproteins (e.g. gA, gB, gC, gD, gE, etc., the nomenclature of gA and gB has been standardized as "gB" by the International Herpes Virus Workshop (Oxford, England) in 1983). Since it has been found that these glycoproteins function as antigens for eliciting an immune response to inhibit HSV infection, various studies have been done on component vaccines comprising these glycoproteins. For instance, Cappel et al. have reported that glycoproteins extracted from HSV-infected cells or virus particles are effective as antigens for preventing infection by HSV (cf. Cappel et al., Arch. Virol., 73, 61, 1982). However, the component vaccine comprising glycoproteins extracted from HSV-infected cells or virus particles contains many proteins originated from the host cells and hence side effects due to the extra proteins are problematic. In order to obtain a suitable component vaccine having no side effects, it is necessary to obtain highly purified glycoproteins. The present inventors directed their research at the gB glycoprotein and have experimentally confirmed that their highly purified gB is a very effective antigen as demonstrated by experimentation in mice [cf. Kino, Cellular Technology, 3, 120 (1984 ).
Glycoprotein gB is usually produced by inoculating a virus into culture cells and then culturing the cells. However, this method is troublesome because it requires the handling of infectious material and involves complicated steps, and further, it is impossible to confirm the complete removal of viral DNA carrying a carcinogenic gene. Thus, it is very difficult to produce a safe component vaccine from natural glycoprotein gB.
The present inventors have found that the desired HSV proteins (i.e. HSVgB) could be produced by a genetic engineering technique, that is, by isolating the HSVgB gene and recombining the isolated HSVgB gene into a specific plasmid vector containing a yeast DNA sequence and an E. coli DNA sequence, wherein said vector contains the expression control region of the repressible acid phosphatase gene of yeast. The HSVgB gene is inserted under the control of the phosphatase promoter and yeast is transformed therewith and cultured to obtain the desired product. (cf. Japanese Patent Application Nos. 151766/1984 and 262465/1984, U.S. Ser. No. 755,776, European Patent Application No. 85109042.3 and Canadian Patent Application No. 487034).
The process disclosed in the above applications can produce very pure HSVgB suitable for the preparation of a vaccine, but it is still desirable to develop an improved process suitable for the production of the desired HSVgB in higher yield.